Very true and a good point. Even crossing "pure" BCM chickens to other BCM the eggs are often lighter than the parent stock for the first generation (from what I have read) because of the complexity of genes/interaction between the genes.That was a simple one that shows 4 genes, blue or white and brown. However brown isn't a simple single gene, there are at least 12 that combine to do it. It turns the numbers astronomical.
This is part of the reason why I like dealing with blue egg layers where there is only 1 (really 2 but the other doesn't exist in the USA or Europe) genes that control blue egg genes - except matte vs shiny shells which does impact color but to a limited extent.